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Since I know that this story is going to turn into flame-fest central, I'm going to try to head things off by interjecting an intelligent conversion about some issues that are on my mind at the moment.

First and foremost, does anyone have a torrent of Minix3? Tanenbaum is a bit worried [google.com] about getting slashdotted. If you've got one seeded, please share.

Now with that out of the way. I don't know if anyone else has tried it yet, but Minix3 is kind of neat. It's a complete OS that implements the Microkernel concepts that he's been expounding on for years now. The upsides are that it supports POSIX standards (mostly), can run X-Windows, and is a useful development platform. Everything is very open, and still simple enough to trudge through without getting confused by the myriads of "gotchas" most OS code-bases contain. Unfortunately, it's still a long way from a usable OS.

The biggest issue is that the system is lacking proper memory management. It currently uses static data segments which have to be predefined before the program is run. If the program goes over its data segment, it will start failing on mallocs. The result is that you often have to massively increase the data segment just to handle the peak usage. Right now I have BASH running with a segment size of about 80 megs just so I can run configure scripts. That means that every instance of BASH is taking up that much memory! There's apparently a Virtual Memory system in progress to help solve this issue, so this is (thankfully) a temporary problem.

The other big issue is a lack of threading support. I'm trying to compile GNU PThreads [gnu.org] to cover over this deficiency, but it's been a slow process. (It keeps failing on the mctx stack configuration. I wish I understood what that was so I wouldn't have to blindly try different settings.)

On the other hand, the usermode servers do work as advertised. For example, the network stack occasionally crashes under VMWare. (I'm guessing it's the same memory problems I mentioned earlier.) Simply killing and restarting dhcpd actually does get the system back up and running. It's kind of neat, even though it does take some getting used to.

All in all, I think it's a really cool project that could go places. The key thing is that it needs attention from programmers with both the desire and time to help. Tossing lame criticisms won't help the project reach that goal. So if you're looking to help out a cool operating system that's focused on stability, security, and ease of development, come check out Minix for a bit. The worst that could happen is that you'll decide that it isn't worth investing the time and energy. And who knows? With some work, Minix might turn out to be a good alternative to QNX.:-)

It seems to me the whole issue boils down to memory isolation. If you always have to pass messages to communicate you have good isolation but costly syncronization of data/state and hence potential performance hits. And vica versa: Linux is prone to instability and security breaches from every non-iolated portion of it.

As I understand it, as a novice, the only way to communincate or syncronize data is via copies of data passed via something analogous to a socket. A Socket is a serial interface. If you think about this for a moment, you realize this could be thought of as one byte of shared memory. Thus a copy operation is in effect the iteration of this one byte over the data to share. At any one moment you can only syncronize that one byte.

But this suggests it's own solution. Why not share pages of memory in parallel between processes. This is short of full access to all of the state of another process. But it would allow locking and syncronization processes on entire system states and the rapid passing of data without copies.

Then it would seem like the isolation of mickrokernels would be fully gained without the complications that arrise in multi processing, or compartmentalization.

As I read this, it seems quite analogous to objects in C++ (or any other OOL). All kernel interfaces could publish the data they want to have public, and hide the data that is private to the implementation of the feature.

I would suggest that this will eventually make its way into kernel systems (just like any other good idea that has come from the programming language fields).

> But this suggests it's own solution. Why not share pages of memory in parallel between processes.

This is precisely what shared memory is, and it's used all over the place, in Unix and Windows both. When using it, you are of course back to shared data structures and all of the synchronization nastiness, but a) sometimes it's worth paying the complexity price, and b) sometimes it doesn't actually matter if concurrent access corrupts the data if something else is going to correct it (think packet collisions).

Still, if you have two processes that both legitimately need to read and write the same data, you probably need three processes. The communication overhead with the third process is usually pretty negligible.

There's even more exotic concurrency mechanisms that exist that don't require copying or even explicit synchronization, but they're usually functional in nature, and incompatible with the side-effectful state machines of most OS's and applications in existence today.

Perhaps I'm mistaken, but Isn't shared memory essentially available to the entire macro-kernel and all it's processes. Something more fine grained like a page based socket would let two processes agree to communicate. They would be sending messages to each other over a very wide channel: the entire page, not some serial socket.Some other process could not butt-in on this channel however, since it's not registered to that socket.

> Perhaps I'm mistaken, but Isn't shared memory essentially available to the entire macro-kernel and all it's processes.

The kernel is the arbiter of shared memory, sure, because that's how it works, by futzing with the VM mappings of processes using it. It's not available to every process in the system though, it still has to ask the kernel for access.

But "communication" over shared memory is exactly how it works -- the size of the channel is the size of the entire shm segment. You write as much data a

"99% of the time when using shm, you use a semaphore, another SysV IPC primitive."This is my opinion, but I had to say it: I personally don't like SysV. There are various ways to synchronize, and each method has advantages and disadvantages, but SysV is at the bottom of the pack if you ask me.

process-shared pthread mutexes and conditions are much faster than SysV, because they usually don't make a system call. A disadvantage of the SysV ipc that process-shared pthread mutexes have too is demonstrated by the

If it's shared memory that I pass to the kernel, and I have an application that doesn't parallelize well, then I'm sending pages off to the kernel quite often. That means the kernel uses a lot of memory.There are two options: the kernel could combine pages, either physically or logically; or the kernel could leave that page open for writing by the application in question.

The easiest way would be for each application to have a page or set of pages for sending messages to the kernel; the application would vie

I think it's because typically the "message" which is meaningful to most application is of larger granularity than a single byte, so it makes sense to instead synchronize around that message. Also, you want to ensure that the semantics of your API and your synchronization match. It makes no sense to preserve integrety without preserving semantics. The best way to do that is to either explicitly make a copy, or to "lease" the structure until such time as you are notified that all necessary work has been d

The Linux v Tanenbaum debate reminds me of the debates I hear at work between those who understand that software is a commercial kludge between conflicting/changing requirements, the limitations of time and abilities of support engineers etc., and those who want to do everything "right", often blowing development budgets, and producing unusable, over-optimised, hard-to-maintain code. I exaggerate the distinction for effect, of course.
Linux has built a system, it works and it's used everywhere. Microkernel

Linux has built a system, it works and it's used everywhere. Microkernels are all niche [...]

One of Tanenbaum's central points is that Linux is not used everywhere. In particular, it's not used anywhere that hard-real-timeness, seriously paranoid robustness (e.g. in those applications where a hardware failure should not result in a reboot) etc are important.

The word "niche" is, much like "legacy", often used in places where a more overt dismissal would rightly be seen as unfair. The fact that Linux c

That is how messages have worked in Mach since its inception. The Microkernel would always send messages by page table manipulation whenever possible. Minix-1 did not work that way (for simplicity) - it just copied the bytes. Someone who has downloaded Minix-3 will have to tell us whether Minix-3 can send "page based" messages.

You're on to something...you are very close to the cache. Why are we "debating" this when the asnwer seems very clear once one takes a step back: They (the kernels) can exist in harmony, each in its own place. Tanenbaum makes a decent showing of examples about where and why micros are used. This isn't a "which is better" argument. This should be a "where is one better utilized than the other in situation X" debate. That flamewar I could tolerate. Bottom line is that neither will replace the other, at least in a timely enough manner that it is worth wasting time over now.

Whoo, there, good buddy. Actually I have seen some pretty entertaining videos of less filling / tastes great cat fights on the internet lately. Now, if someone wants to post videos of supermodels catfighting over microkernel / linus, I would then get pretty excited over the whole debate.Wait a minute, too much information here...

Pardon me? Sir? Sir? You seem to have diarrhea of the mouth and constipation of the brain.

Minix 1 & 2 codebases are indeed older than Linux. And they could have *been* Linux, except that Tanenbaum was focused on teaching. As a result, he rejected the requests to add features, thus leading to the development of Linux.

However, he has apparently decided that it's time to start a microkernel p

You-betcha. I honestly think Mr. Tanenbaum is wasting his time in replying to Slashdot. If the last article proved anything, it's that the majority of responders were stuck on the whole "Linus 'won' this over a decade ago, so STFU!" (No one really 'won' the argument, but that's beside the point.)

There were a couple of good replies in there, but they all got drowned out in the noise. Soooo, I think it's a better idea to focus on how Minix might be made a viable OS rather than arguing the same nonsense all over again. As several of the posters here have already proven, they're not reading Tanenbaum's arguments anyway. So why should we expect this time be any different than the last?

"Linus 'won' this over a decade ago, so STFU!"Hey Linus did win this. He was right and NOTHING has changed in the last ten years!Computers are not that much faster than they where back then and the need for security is no different that then!Yes I am so kidding. Linus won this because at the time his goal was to get out a Unix clone that ran on the 386 as quickly as possible. Doctor Tanenbaum on the other hand was interested in a Unix clone that would run on cheap hardware and that made a very good learning tool. For his goal Minix was the better system.Now we live in world of Gigs. It is common to have many gigs of hard drive space, at least a gigabyte of ram, and multigigahertz multi-core cpus. Not to mention that even the cheap built in graphics chip sets would blow the doors off of any video card you could get in 1995.For all but the biggest FPS gaming freak our computers are fast enough. What we want now is reliability, security, and ease of use. I use Linux every day. I depend on Linux. What I will not do is give up hope on something better than what we are using today. New idea's should be explored.

I am also a little bit disapointed how little respect Doctor Tanenbaum has gotten on Slashdot. Linus compiled the first versions of Linux using Gcc running under Minix. I am pretty sure that Linus read Doctor Tanenbaum's book and probably learned a lot about how to write an OS from it. When it comes to computer science Tanenbaum's name is right up there with Wirth and Knuth. Of course the odds that any of the people that use STFU in a post have ever read Knuth, Wirth, or Tanenbaum is probably not worth measuring.Even if you are not convinced that Tanenbaum's methods are correct, his goals of a super reliable, self-healing, and secure OS are correct.

So **PLEASE** no more comments like "If Tanenbaum thinks microkernels are so great, why doesn't he write an OS based on one?" He did.

i don't reall know what you mean by proof of concept

again, FTFA:

It is definitely not as complete or mature as Linux or BSD yet, but it clearly demonstrates that implementing a reliable, self-healing, multiserver UNIX clone in user space based on a small, easy-to-understand microkernel is doable. Don't confuse lack of maturity (we've only been at it for a bit over a year with three people) with issues relating to microkernels.

i know this is slashdot, and RTFA is some kind of mortal sin, but please at least try.

Tanenbaum wrote (in TFA):The average user does not care about even more features or squeezing the last drop of performance out of the hardware, but cares a lot about having the computer work flawlessly 100% of the time and never crashing. Ask your grandma.

Interesting. My mom recently bought a computer for my grandma. Grandma doesn't have a problem with the computer crashing at all. Her secret? She never turns it on.

When did we collectively forget that everything has its place...I doubt I'll ever see anything but a monolithic kernel on my desktops. No different than any given OS having its place. Windows and Ubuntu (until something better) will live on my desktops, not on my server. Why can't we just all get along?

Have you read the article? Tanenbaum basicly starts out by saying this is not a 'fight', but a technical discussion. Communication and debate is an important part of research and development. That's what is
being attempted here, at least at face by Tanenbaum. There may be antagonism behind the scenes, or bias in presentation, but that is just human. The primary intent is to advance the state of the art, not fight.

All this 'what's the point' or 'we have this now' type of talk really bugs me. Everything can always be improved, or at least that is the attitude I'd like to stick with.

> When did we collectively forget that everything has its place

Another key component of research and development is to question everything. Not throw everything away and always start over, but to at least question it. Just because monolithic kernels rule the desktop now does does prove that monolithic kernels are inherently the best desktop solution.

In effect it is sometimes good to not even recognize a notion of 'everything has its place'.

And v3.12 (I think, I'm going from memory here) will finally support the X windowing system

That's odd. I could have sworn that I was just using an X-Terminal on it a few minutes ago.

Oh wait. I was using an X-Terminal. How in the world did that happen? </mock-sarcasm>

To be fair, getting X-Windows running is a recent development. On the other hand, the entire Minix3 codebase is a recent development. (Only a half-year old.) They're moving at a pretty good clip for a brand-new OS.:-)

Linux is very reliable for me, even on newer hardware with a bleeding edge kernel. Why should I care whether it has a microkernel or monolithic kernel? Everything I deal with is user space. If it runs GNOME, is POSIX-like, and supports some kind of automatic package management, I'll be happy as a clam.

Will hardware drivers be developed faster and more reliably with a microkernel? That seems to be the biggest hurdle in reliable OS development these days... Anyone have a good answer for that, I honestly don't know.

Linux as a kernel is so reliable for me that the only times I have to use the reset button are when hardware malfunctions (usually something that a microkernel can't help with, like RAM, CPU, or the video card, though in the latter case, I tend to ust leave the computer running and ssh in from elsewhere...).I noticed two things about Tannenbaum's piece though. Essentially all of the microkernels he listed were either used in dedicated (including embedded) systems or were not true microkernels by his own ad

I'm doing a (free) operating system (just a hobby, won't be big andprofessional like gnu) for 386(486) AT clones. This has been brewingsince april, and is starting to get ready. I'd like any feedback onthings people like/dislike in minix, as my OS resembles it somewhat(same physical layout of the file-system (due to practical reasons)among other things).

I've currently ported bash and gcc, and things seem to work.This implies that I'll get something practical within a few months, andI'd like to know what features most people would want. Any suggestionsare welcome, but I won't promise I'll implement them:-)

I have never experienced the "stalling" problem that affected a very small number of 2004 and 2005 Priuses last year. (OK, hubris correction, make that "not yet..." although my car's VIN is outside the range of VINs supposedly affected).

It was apparently due to a firmware bug.

In any case, when it happened, according to personal reports in Prius forums from owners to whom it happened, the result was loss of internal-combustion-engine power, meaning they had about of mile of electric-powered travel to get to a safe stopping location. At that point, if you reset the computer by cycling the "power" button three times, most of the warning lights would go off, and the car would be fine again. Of course many to whom this happened didn't know the three-push trick... and those to whom it did happen usually elected to drive to the nearest Toyota dealer for a "TSB" ("technical service bulletin" = firmware patch).

These days, conventional-technology cars have a lot of firmware in them, and I'll bet they have a "reset" function available, even if it's not on the dashboard and visible to the driver.

And even if you lumped them into cars, so, you have what, a few hundred prius's that have reset buttons, among the hundreds of millions of cars. And every computer in existance still has a reset button, and at some point in time that reset button has been exercised.

...so I can't spend a lot of time in dicussing this, but I always that the main benefit of micro-kernels is completely wasted unless you actually have utilities that can work in partially-functioning environments. What good is it to be able to continue to run a kernel even with your SCSI drive disabled, if all your software to fix the problem is on the SCSI drive?

Now in theory I could see a high-availability microkernel being a good, less expensive alternative, to a classic mainframe environment, especially if you had a well written auto-healing system built in as a default. But that would require a lot of work outside the kernel that just isn't being done right now. And until it is, micro-kernels don't have anything more to offer than monolithic kernerls.

To put it in API terms - it doesn't matter very much whether your library correctly returns an error code for every possible circumstance, when most user level code doesn't bother to check it (or just exits immediately on even addressable errors).

Its a chicken/egg situation. Until the underlying mechanisms needed for self-healing are there, we won't get self-healing systems. Until the user space code for self-healing is there, nobody thinks its worthwhile to support self-healing mechanisms. Thankfully a few folks realize that if they build it, people will come.Also, your API metaphor is a little bad. While you're right about the end result, saying that this invalidates the utility of the API is wrong imho. The advantage of having the API remains, be

QNX is everywhere, you just don't realize it. ATMs run it, lots of medical equipment runs it, lots of other embedded apps that you don't even think of run it.

The examples Andy cites prove that in fact the microkernel concept has won in every single field where stability has gone beyond being something people wanted to something they demand. As soon as the general public realizes computers don't HAVE to crash, they'll win there too.

"It certainly can do. A bolt which is used on buses around the world is in a niche compared to, say, windscreen glass."While I see your point, and agree to an extent, its a poor metaphor (windscreen glass is a pretty niche application of glass, wouldn't you say?).

My point was to refute the implied "QNX isn't anywhere important" statement rather than the exact meaning of niche.

"But MY computer never crashes (Linux); so what else has it to offer? Security? Got that too."Thats wonderful, and my data center ful

All of these ideas are old, and while high performing don't address the largest issue of all, cross kernel compatability.

Sure you can recompile and all that jaz, but I'd love to see a day where an app could run on any number of kernels out there. This creates real competetion.

What I'd like to see if a kernel more like a CPU. Instead of linking your kernel calls, you place them as if you where placing an Assembly call. Then we can have many companies and open source organizations writing versions of it.

As we move towards multi core cpus this could really lead to performance leads. Where one or more of many cores could be dedicated to the kernel operations listening for operations and taking care of them. No context switches needed, no privledge mode switching.

Drivers and everything else run outside of kernel mode and use low level microcode to execute the code.

The best part I think is you could make it backword compatiable as we re-write. A layer could handle old kernel calls and change them to the micro codes.

As we define everything more and more then we might even be able to design CPUs that can handle it better.

> Virtually all of these postings have come from people who don't have a clue what a microkernel is or what one can do.

Okay, I spent 2 years working as a engineer in the OSF's Research Institute developing Mach 3.0 from 1991. Let me answer Linus's question in a simple fashion. What Mach 3.0 bought you over Mach 2.5 or Mach 2.0 was a 12% performance hit as every call to the OS had to make a User Space -> Kernel -> User Space hit. This was true on x86, Moto and any other processor architecture available to us at the time. Not one of our customers found this an acceptable price to pay and I very much doubt they would today. One of the reasons Microsoft moved a lot of functionality into the Kernel between NT 3.5 and NT4.0 was performances (NT being, at its origins a uK based OS).

What of the advantages ?

Is porting easier? No not really, the machine dependent code in Mach 2.5 and Mach 3.0 was already well abstracted.

You could run two OS personalities at once, for example you could have an Apple OS and Unix running at the same time. But why would any real world clients want to do this?

Problems in the OS personality wouldn't bring down the uKernel - but they might stop you doing any useful work while you reboot the OS personality.

Other things like distributed operating systems (and associated fault tolerance) were perhaps aided by the uK design and this is a path that, in my humble opinion, the OSF should have pursued with greater zeal than they did. Back in 1991 we had a Mach 3.0 based system that ran a uK across an array of x86 nodes but had different parts of the OS - say IO or memory management running on different nodes. From a user standpoint all the machines (in reality bog standard 386 machines linked by FDDI) looked like a single computer running a Unix like OS.

I remember discussing Linux with my colleagues back in 1993, some were impressed and thought the nascent OS model was very powerful, others dismissed it as a toy with no real future. I suspect Tannenbaum was also amongst the poo=pooers and has become pretty annoyed about how things have turned out.

Okay, I spent 2 years working as a engineer in the OSF's Research Institute developing Mach 3.0 from 1991. Let me answer Linus's question in a simple fashion. What Mach 3.0 bought you over Mach 2.5 or Mach 2.0 was a 12% performance hit as every call to the OS had to make a User Space -> Kernel -> User Space hit. This was true on x86, Moto and any other processor architecture available to us at the time. Not one of our customers found this an acceptable price to pay and I very much doubt they would tod

'Way back when we read the first rev of this discussion, Tanenbaum made good points. At the same time, Linus was able his little monolithic kernel project jump through the hoops he wanted it to.

Years later, Tanenbaum still makes valid observations, Linus and others continue to make a rather larger project jump through the hoops, and that's fine. The results of academic research may or may not get traction outside of a university, but without the research, there wouldn't be alternatives to contemplate. If I've gathered nothing else about Linus' personality from his writings over the years, it's that he seems to be practical, not particularly hung up on architectural (or licensing) theories... unlike me.

At some point, if his current architecture just isn't doing it for him any more, he might morph into Tanenbaum's 'A' student. It won't be because a microkernel was always right, but that it was right now.

Getting the architecture of a microkernel right is really hard. There are some very subtle issues in how interprocess communication interacts with scheduling. If these are botched, performance under load will be terrible. QNX got the performance part right. Mach got it wrong. Early Minix didn't address this issue. See this article in Wikipedia [wikipedia.org]. Other big issues include the mechanism by which one process finds another, and how mutually mistrustful processes interact. If you botch the basic design decisions, your microkernel will suck. Guaranteed.

Most academic attempts at microkernels have been duds. One can argue over why, but it's the commercial ones, like QNX, VM, and KeyKos that work well, while the academic ones, like Mach, EROS, and the Hurd have been disappointing.

Security models really matter. And they're hard. Multics got this right. KeyKos got this right. QNX is no better than UNIX in this area.
Designers must work through "A can't do X, but A can trick B into doing X" issues.

Trying to turn a monolithic kernel into a microkernel doesn't work well. Mach, which started life as BSD UNIX, ran into this problem, which is why MacOS X isn't based on the microkernel version of Mach.

Drivers in user space have real advantages. Not only is the protection and restartability better, but because they have access to all the regular user program facilities, drivers for more modern devices are much easier. Things like Firewire and USB device discovery and hot-plugging reconfiguration are far easier at the user level, where you have threads, can block, and can call other programs. The old "top half and bottom half" driver approach doesn't generalize well to today's more dynamic configurations. Monolithic kernels have had to add kernel threads and dynamic loading of modules to handle all this, resulting in kernel bloat.
Of course, a big advantage of less-privileged drivers is blame management - you can tell whether the OS or the driver is at fault.

Startup requires more attention. A microkernel often doesn't contain the drivers needed to get itself started. So the startup and booting process is more complex. QNX has a boot loader which loads the kernel and any desired set of programs as part of the boot image. This gets the console driver and disk driver in at startup, without having to make them part of the kernel.

The performance penalty is real, but not that big There's a performance penalty associated with the extra interprocess communication. It's usually not that big, but there are areas where it's a problem. If it takes one interprocess call for each graphics operation, for example, performance will be terrible. NT 3.51 had a nice solution to this problem, designed by Dave Cutler. (NT 4 and later have a more monolithic kernel, but that had to do more with making NT bug-compatible with Windows 95 than with performance problems.)

I/O channels would help IBM mainframe channels, which have an MMU between the peripheral and main memory, are better suited to a microkernel architecture than the "bus" model the microcomputer world uses. In the mainframe world, the kernel can put program in direct communication with the hardware without giving it the ability to write all over memory. So there's little penalty for having drivers in user space. Which is why VM for IBM mainframes has been doing this for decades.

If you get it right, the kernel doesn't change much over time. This is the big win, and why microkernels become more stable over time. In the QNX world, USB and Firewire support were added with no changes to the kernel. (I wrote a FireWire camera driver for QNX, so I'm sure of this.)
The IBM VM kernel has changed little in decades.

Before we get into arguments or understanding arguments, two most important things to note:- AST is a prefessor. His interest in doing research and building the best systems for the *future* that he believes in.- Linus is an engineer. His interest is building a system that works best *today*.

We simply need both. Without pioneering work done before in other OSes (this included failures), Linux wouldn't have been like this today. The greatest reason for its success it not it's doing something cool, but it's doing things that are proven to work.

So who is right? I'd say both. Linus has said this in 1992: "Linux wins heavily on points of being available now."

Linus admits microkernels are "cooler", but he didn't (doesn't) believe in it *today* because none of the available microkernels could compete with Linux as a *general purpose* OS. It's funny how AST listed "Hurd" as one of the microkernels - it totally defeats his own arguments. The fact is Hurd is still not available today despite it was started before Linux.

Many people talk about QNX. Sure, in many cases (especially mission critical, RTOS, where reliablility is so much more important than performance and usability) microkernels are better, but we really shouldn't compare a general-purpose OS with real-time or special purpose OS.

So we go back to the old way: code talks. So far microkernel proponents keep saying "it's possible to do microkernel fast, etc" but the fact is they have never had an OS that could replace Linux and other popular OS that everybody could run on their desktop with enough functionality. There are two possible reasons:1. Lack of developers. But why? Do people tend to contribute to Linux because Linus is more handsome (than Richard Stallman that is)? There gotta be some reasons behind it other than oppotunities right?2. Monilithic kernels are actually more engineerable than microkernels, at least for today.

Linux as a kernel is sufficiently mature that the problems Linus is spending time on are management and scalability problems--organizing the large-scale kernel hacking effort and dealing with massively parallel processing.

I'd like to see Linus say "I've done a monolithic kernel and proven its success. Now I'm going to build a performant microkernel and see what all the fuss is about." He could hand over Linux kernel development to the senior crew that's already taking care of the major modules, and try something else.

Essentially, it would be cool for someone like Linus, with his incredibly strong practical engineering bent, to do again what he did with Linux: semi-clean-sheet a new kernel that meets his performance requirements, but is designed around different strategies for achieving what every OS tries to achieve.

I bet that, in two or three years, he would recant his earlier dismissal of microkernels and say that there's actually some interesting stuff there, and along the way solve some of the perennial complaints that slashdotters always bring up whenever microkernels are mentioned. In his heart of hearts, I'm sure Linus has some legacy issues with the current kernel design that he'd love to jettison, but can't without massively re-organizing the existing architecture, in which too many interested parties are already involved.

And he could put Stallman and the HURD boys to shame *again*, which is a twofer:)

More important than micro/macro to me would be the ability to keep the system running and edit the system. I used to do that with Scheme back in my college days. It made me realize how something like the telephone system could keep running 24/7 and never go down. These days with MS Windows I gotta reboot every 30 days, and the same with these fscking Linux kernel updates. What if I don't ever want to reboot. I think a microkernel/interpreter would let you modify the running system a lot easier. You could even make incremental changes and then check to make sure they work - preserving the old code so a rollback would be simple.

The point that Andy makes which I agree on, is that computer software is still in its infancy. The part I disagree with is that it'll change by him stating the obvious.

Has anyone thought of that the fact this very conversation may go down in the history of computer science? In 30 more or less years, lecturers will be telling their students about this argument! We're witnessing a more interesting slice of history than our normal mundane day lives:)

There is a simple reason why microkernels do not work in practice: the abstraction layer is in the wrong place.

<simplification>A hardware driver doing output has to take raw bytes from a process, which is treating the device as though it were an ideal device; and pass them, usually together with a lot more information, to the actual device. A driver doing input has to supply instructions to and read raw data from the device, distil down the data and output it as though it came from an ideal device.</simplification>

In general, the data pathway between the driver and the process {which we'll call the software-side} is less heavily used than the data pathway between the driver and the device {which we'll call the hardware-side}.

<simplification>In a conventional monolithic kernel {classic BSD}, a hybrid kernel {Windows NT} or a modular kernel {Linux or Netware}, device drivers exist entirely in kernel space. The device driver process communicates with the userland process which wants to talk to the device and with the device itself. All the required munging is done within the kernel process.
In a microkernel architecture, device drivers exist mainly in user space {though there is necessarily a kernel component, since userland processes are not allowed to talk to devices directly}. The device driver process communicates with the ordinary userland process which wants to talk to the device, and a much simpler kernel space process which just puts raw data and commands, fed to it by the user space driver, on the appropriate bus.</simplification>

Ignore for a moment the fact that under a microkernel, some process pretending to be a user space device driver could effectively access hardware almost directly, as though it were a kernel space process. What's more relevant is that in a microkernel architecture, the heavily-used hardware-side path crosses the boundary between user space and kernel space.

And it gets worse.

<simplification>In a modular kernel, a device driver module has to be loaded the first time some process wants to talk to the device. {Anyone remember the way Betamax VCRs used to leave the tape in the cassette till the first time the user pressed PLAY? Forget the analogy then} which obviously takes some time. The software-side communications channel is established, which takes some time. Then communication takes place. The driver stays loaded until the user wants it removed. Then the communication channel is filled in and the memory used by the module is freed, which obviously takes some time.

In a microkernel architecture, a user space device driver has to be loaded every time some process wants to talk to the device. The software and hardware side communications channels have to be established, which take some time. Then communication begins in earnest. When that particular process has finished with the device, both channels are filled, and the memory used by the driver is freed; which takes time. Between this hardware access and the next, another process may have taken over the space freed up by the driver, which means that reloading the user space driver will take time.</simplification>

It makes good practical sense to put fences in the place where the smallest amount of data passes through them, because the overheads involved in talking over a fence do add up. That, however, may not necessarily be the most "beautiful" arrangement, if your idea of beauty is to keep as little as possible on one side the fence. It also makes sense for device drivers which are going to be used several times to stay in memory, not be continuously loaded and unloaded. {Admittedly, that's really a memory management issue, but no known memory manager can predict the future.}

If you have a computer science degree you have probably used at least one if not more of his textbooks.

His book on computer organization was one of the worst pieces of crap I've ever been forced to drop cash on. He frequently goes off on tangents and spends more time trying to be clever than clearly explaining the material. I fucking hate Tanenbaum, even though I like micro-kernels.

I know you're being facetious (comon, mod points for the SAT word), but for those who don't know, Andrew Tanenbaum is covered at Wikipedia [wikipedia.org]. His textbook, Modern Operating Systems, is probably one of the most widely used and excellent resources on the subject. He also likes to get into flame wars with Linus Torvalds when he gets bored. This is ironic because supposedly Linus used Tanenbaum's Minix as a starting point and influence for Linux.

He also likes to get into flame wars with Linus Torvalds when he gets bored.

Really? And what exactly do you base this on? According to the article, which it's clear that you did not read, Tanenbaum simply had a recent article printed in IEEE Computer and someone on Slashdot posted a link to it, which caused Linus to weigh in with his 2 cents about something that was never directed at him. It sounds more to me like Linus is obsessed with proving that macrokernels are the only way to go. Why does he even care? It's not like Minix is a threat to Linux. If he believes so strongly that microkernels are wrong, he should just let Tanenbaum and company waste their time on them instead of endlessing arguing the same points he made years ago.

He created Minix, the OS Linus Torvalds (the creator of the Linux kernel, which is the 'kernel' of an operating system, which if you need to have that explained, please leave slashdot and never return) used before and during his creation of Linux. Tanebaum and Torvalds had a long flamewar over microkernel vs monolithic kernels back in the early days of Linux development (you can find it with a bit of searching, its pretty entertaning, especially the parts where Tanebaum predicts the future).Minix was an Mi

No, actually he created two that I know of. Well, technically three since MINIX 3 is probably sufficiently different from MINIX 1 to be thought of as a different kernel. Amoeba was another microkernel-based OS designed to run on distributed systems, presenting an entire cluster as a single machine.

MINIX 1 was a teaching tool. MINIX 3 is a real OS, although still very young (less than two years old), but doing very well. Amoeba is so far ahead of Linux conceptuall

I think the real issue here that brings this argument full circle is that when speed for basic operations was an issue then Linux wins. But in todays world we have plenty of fast CPUs. What we lack is stability and the ability to architect robust systems in a scalable manner. For applications that require every ounce of speed, well only those that actually interact with the OS will notice much differnce between Micro-kernel and linux. So this is not to say that the whole computer is slow under micro ke

That's a very good point, and one that people keep forgetting. If microkernels are so great, where are they? Let's take a look at notable microkernels:* QNX Neutrino. This is the most successful microkernel ever. It deserves all the praise it gets. Yet it is still a niche product.

* Hurd. After twenty years we're still waiting for a halfway stable release. Hurd development is almost an argument *for* monolithic kernels!

*Minix. This is still an educational kernel. A teaching tool. It remains unsuitable for "r

*Minix. This is still an educational kernel. A teaching tool. It remains unsuitable for "real world" use.

Actually, it's a start of a full-up Microkernel operating system. This isn't your grand-pappy's Minix, it's a brand new code base under the BSD license, intended to be developed out into a complete system. It's still taking baby-steps at the moment, but it's coming along quite nicely.

* NT. This is NOT a microkernel!

NT is a hybrid. It has Microkernel facilities that are constantly being used for something different in each version. Early versions of NT were apparently full Microkernels, but this was changed for performance.

* QNX Neutrino. This is the most successful microkernel ever. It deserves all the praise it gets. Yet it is still a niche product.

I would hardly call QNX a "niche" product. Running on everything from your car engine to Kiosk PCs (yes, that stupid iOpener ran it too), it's an extremely powerful and versatile operating system. Its Microkernel architecture even gives it the ability to be heavily customized for the needs of the application. Don't need networking? So don't run the server! Need a GUI? Just add the Graphics server to the startup.

Microkernels haven't failed. However, you may notice that nearly all the popular Operating Systems we use today were all developed back in the late 80's and early 90's. The real problem is that there hasn't been a need to develop new OSes until now. Now that Security and Stability are more difficult pressing issues than performance, we can go back to the drawing board and start designing new OSes to meet our needs for the next decade and a half.

NT is a hybrid. It has Microkernel facilities that are constantly being used for something different in each version. Early versions of NT were apparently full Microkernels, but this was changed for performance.

No-no-no-no-NO! I swear this kills me... Why does this myth continue to propogate? The ONLY thing about NT that was EVER uKernelish was that it did alot of IPC (message passing) and that it implemented "personalities" (but it did so in a most decidedly non-microkernel way). Both of these traits were commonly associated with microKernels at the time, but regardless the things that ACTUALLY make a kernel a microKernel never existed in NT... EVER...

All drivers that touched an I/O port HAD to be implemented in kernelmode. That restriction goes back to the original NT 3.1 release and was NEVER otherwise.

Although filesystems are modular to a certain degree, the nuts and bolts of all filesystems have to be implemented in kernelspace.

While initially GDI device drivers (i.e. graphics and printing) were implemented in userspace, this concept was thrown out in NT4. Btw, there was nothing especially microkernelish about this; X is implemented in a similar way to the pre-NT4 GDI as far as that goes. Graphics and printing after all are not generally an esential function of an OS from a functionality perspective.

A real challenge means that the "someone" (the community) should define the minimum requirements to be matched.
For reliability, effectiveness and efficiency.
Maybe one can even define a way to measure these features, so the winner would be declared by measurement!

If you were given the choice between rebooting your machine every 3 months or so for updates/driver install or never rebooting your machine and but taking a 3-5% performance hit (I think this is what the most efficient uKernels waste on address space switches) which would you choose.

I know my answer. For embedded systems/media center type stuff I don't care about the 3-5% performance hit. I don't ever want to screw with them.

For my computer I don't care about rebooting every 3 months or so. I want that extra little bit of speed.

You would be one of those uninformed pontificators Andy so eloquently railed against.

"For small embedded environments where speed or device support isn't a main concern. Micro-kernels will excel for their stability but take a look around and that's not reality or what we have today. We have lots of different hardware, lots of different interfaces and to manage that all via objects it'll just be extremely large."

And none of that has anything to do with monolithic versus microkernel, except perhaps tangentially. Microkernels do not ask each device driver to be a server all its own with zero code reuse, they use generic servers to wrap drivers for specific hardware while still isolating them from kernel space. This means there's no functional difference to the driver programmer from a monolithic to a microkernel architecture, either way you look at the driver interface and write the necessary code.

"If you think the linux kernel is big the relevant code for this would be numerous times larger. It just pushes the code from the kernel into userspace and you will definitely need more code to manage and access data structure"

Why do you suddenly need more code to the same thing? Andy's point is that when you stop sharing data structures, and instead start passing messages from one discrete server to another through well defined interfaces you reduce the amount of complexity (and therefor code) involved in protecting the coherency of those data structures. You will end up with more interfaces, but thats not necessarily a bad thing. I'd gladly trade all of the critical section protection logic for some nice interface logic. Especially since making the latter work reliably is a hell of a lot easier to do, and gives each subsystem the freedom to rework their internals without requiring me to lift a finger.

"If you can isolate your facets and only plan on supporting X number of devices/platforms/chipsets/etc and don't expect any blazing performance. Microkernels are great. Beyond that? With the rate that technology moves, it just becomes a management nightmare."

There's still no credible evidence to suggest that microkernel performance is that horrible, especially with modern clock speeds. Aside from gaming and large scientific compute clusters, very little being done today on a computer uses any significant measure of their speed. We've already covered how you're totally off base on device support (i.e. its orthogonal to the entire debate), and you throw "management nightmare" out there without bothering to define it, let alone defend it.

Large unix systems are already complex as hell to manage. A lot of that complexity is "hidden" in the kernel, which while fine for desktop users is a big pain for system administrators, and would be exposed for manageability in a microkernel setup.

As for OS X and its performance, its not horribly slow. Especially considering that your complaint almost certainly centers around PPC performance not x86, where it was hampered by lower clock speeds that were not counterbalanced by better IPC in any significant fashion. OS X's memory hunger has little to do with the kernel and lots to do with their operating environment, and all of the gee whiz graphical functionality that OS X brings along with it.

Ultimately though, OSX performance is a success story because on a G3 700mhz with 256M of ram its actually useable. Have you tried running Windows XP on a similar setup? Tried turning all of the eye candy on? Bet you didn't like the way it performed either.

What do you mean by ``good performance''? Will an arbitrarily chosen microkernel run on a 3GHz Opteron as fast as, say, SunOS 3.0 on a 15Mhz 68020 with 4MB of RAM? Clearly it will. And that was pretty fast at the time. What performance hit is acceptable in exchange for reliability is a difficult question, but in a lot of spaces a 90% hit would be acceptable, and I can think of applications where a 99% hit would be acceptable if the microkernel did indeed deliver the reliability that's claimed. After all, running at only 25% of the potential performance (and no-one's claiming that's the hit) is only 3 years on Moore's law. Vista's how many years late?

More to the point, ``because it's faster'' has been the bane of Unix. To see that in stark relief, look at the shambles of NFS being in the kernel. Rather than fix the generic problems of providing a user-space nfsd, we saw a race into the kernel for a cheap my-code-only win, plus the horror of system calls that never return. Look at the vogue for in-kernel windowing systems (Suntools, for example) although X mercifully killed that off. Repeatedly we've seen massively complex and invasive kernel subsystems produced, when a generic solution to the problems that going into the kernel allegedly solves would have benefitted everyone for longer.

You've got a problem. You decide to solve it with a kernel extension. Now you've got two problems.

On the microkernel side we have Minix 3, 15 years after the first not-really-open-source-but-code-available microkernel UNIX systems.

On the monolithic kernel side we have... what? 15 years after the first not-really-open-source-but-code-available monolithic kernel UNIX systems we had... hmmm... things like MiNT, and bits of BSD, but even Net/1 was a few years in the future and Minix wasn't even out.

I think, after you allow for the 20 year head start, microkernels aren't doing that badly.

You don't time things when code is available, you time things when people are available to code. For both micro- and macro- kernels, the race started with the 80386 and the affordability of 32-bit CPUs for the average developer. The reason you didn't have Free Software kernels 15 years after the quasi-availability of UNIX source was that hardly anyone could afford the hardware. The ability to collaborate over a network also made a huge difference.And Minix 3 doesn't count for real world use. It may be a goo